The Unrestricted Frequency Changer, also called Unrestricted Frequency Converter, is known as a static "artificially" commutated frequency converter with variable output voltage. The UFC has been described in the context of a variable speed AC motor drive in U.S. Pat. Nos. 3,470,447 and 3,493,338 of Gyugyi.
The Unrestricted Frequency Changer, when compared to other static power converters, has significant advantages that make it particularly suitable for providing variable frequency electric power to control the speed of ac motors. These advantages are as follows:
1. Single stage power conversion with bidirectional power flow (i.e., power can flow either to, or from, the load). This permits regenerative braking of the motor.
2. A wide output frequency range, not limited by the input (supply) frequency. Therefore, the generated output frequency can be lower, higher, or equal to the input frequency.
3. The frequency spectrum of the output waveform is independent of the amplitude of the wanted fundamental component. Furthermore, the frequencies of the "unwanted" (harmonic) components in the output waveform are widely separated from the fundamental frequency over the total output frequency range. The separation of the harmonic frequencies from the fundamental increases "naturally" (i.e. without changing the method of output voltage waveform construction) as the fundamental output frequency decreases. Thus, the frequencies of the harmonic currents in the motor remain high relative to the fundamental, even at low speeds so that the motor runs without cogging.
4. The lagging (inductive) motor displacement (power) factor results in leading (capacitive displacement (power) factor (with equal phase angle) at the ac supply. Unity output displacement factor is reflected back to the ac supply without change.
5. Simple control. The output frequency and voltage can be controlled by two appropriately displaced pulse trains, both having the same even rate.
The prior art on Unrestricted Frequency Changer motor drives is described in the afore-stated U.S. patents. It involves three identical bidirectional converter power circuits supplying the three stator windings of the motor, with a gating logic generating the electrical signals to turn on and off the bilateral switching devices of the power converters, while pulse trains in response to external analog signals determine the output frequency and voltage applied to the motor. These two pulse trains are so coordinated that the output voltage increases with increasing output frequency in order to maintain an essentially constant air-gap flux in the motor.
The present invention resides in a topology for the bridge version of the unrestricted frequency changer which allows a reduction of the number of bilateral switches necessary in the operation and control of the converters. In this respect advantage is taken from the situation where the output voltage is symmetrically controlled, as the case is, for instance with an AC motor drive.